Cook the potatoes in the oven until tender. Mash with the butter, cream, and truffle oil. Set aside. Make the shortcrust. Cook the onions down in some butter until browned, sweet, and delicious. Roll the crust out really thin and lay in a lightly greased tart pan. Add the potatoes, followed by the onions and blue cheese. Bake at 425 for about 20 minutes until the crust is cooked. Drizzle with some more truffle oil before serving.